Clean method for preparing layered double hydroxides

ABSTRACT

Disclosed is a clean method for preparing layered double hydroxides (LDHs), in which hydroxides of different metals are used as starting materials for production of LDHs by atom-economical reactions. The atom efficiency of the reaction is 100% in each case because all the atoms of the reactants are converted into the target product since only M 2+ (OH) 2 , M 3+ (OH) 3 , and CO 2  or H n A n−  are used, without any NaOH or other materials. Since there is no by-product, filtration or washing process is unnecessary. The consequent reduction in water consumption is also beneficial to the environment.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application claims priority from Chinese PatentApplication No. 200710062650.9, filed on Jan. 12, 2007.

TECHNICAL FIELD

The present invention relates to a clean method for preparing layereddouble hydroxides, and belongs to the field of synthesis of inorganicnonmetallic functional materials.

TECHNICAL BACKGROUND

Layered double hydroxides (LDHs), which belong to a typical family ofanionic layered materials, can be represented by the general formula[M²⁺ _(1-x)M³⁺ _(x)(OH)₂]^(x+)(A^(n−))_(x/n).mH₂O, where M²⁺ is abivalent metal cation, M³⁺ is a trivalent metal cation, A^(n−) denotesan interlayer anions with negative charge n, m is the number ofinterlayer water molecules, and x is the molar ratio of the M³⁺ to thesum of the M³⁺ and M²⁺. The identity and ratio of the layer elements aswell as the interlayer guest anions can be adjusted over a wide range inorder to obtain materials with specific structures and properties.Because of their flexible composition and versatility, LDHs have beenwidely investigated for their potential applications in the fields ofcatalysis, adsorption, ion exchange and functional materials.

LDHs are traditionally synthesized by coprecipitation methods,hydrothermal methods, ion-exchange methods or calcination-rehydrationmethods. A large amount of sodium salt is produced as by-product duringthe preparation of LDHs by traditional methods. The sodium salt motherliquor is always discharged directly due to the high energy costs ofevaporation, and thus leads to environmental pollution. In addition, theuse of strong alkali in the synthesis process means that the productmust be well washed with water (tens or even hundred times of theproduct's mass), which leads to significant waste of water and problemswith treatment of the alkaline effluent. Thus it is necessary to developan environmentally friendly technology for preparation of LDHs.

The coprecipitation method is the most popular method used to prepareLDHs. A mixed salt solution containing the metal ions which constitutethe layers are coprecipitated with alkali in order to obtain the LDHs.Either the mixed salt solution or the alkali solution can contain thecorresponding interlayer anionic group. In a related reference (Y. Zhao,F. Li, R. Zhang, D. G. Evans, X. Duan, Preparation of layereddouble-hydroxide nanomaterials with a uniform crystallite size using anew method involving separate nucleation and aging steps, Chem. Mater.,2002, 14:4286-4291), a method for synthesis of LDHs has been reported,in which LDHs are prepared by coprecipitation of a mixture of solublesalts of bivalent and trivalent metal ions with Na₂CO₃ and NaOH.However, in this method, large quantity of water is required to wash theproduct after the reaction, due to the large amount of sodium saltsproduced in the reaction and the strongly alkaline solution, and asignificant waste of water is thus caused.

The hydrothermal method for preparation of LDHs is a method in whichinsoluble oxides or hydroxides containing the metal ions to beincorporated in the layers are treated with water at a high temperatureunder a high pressure. In this method, Na₂CO₃ or NaHCO₃ is generallyused as main starting materials, and the sodium salt formed as aco-product needs be removed by washing which causes a lot of waterwaste. In a related reference (Zhi Ping Xu, Guo Qing (Max) Lu,Hydrothermal Synthesis of Layered Double Hydroxides (LDHs) from MixedMgO and Al₂O₃: LDH Formation Mechanism, Chem. Mater. 2005,17:1055-1062), a process for preparing MgAl—CO₃ ²⁻-LDHs has beenreported, in which Na₂CO₃ or NaHCO₃ is added into a mixture of MgO andAl₂O₃ at 110° C., and the product contains a lot of Na⁺ which needs tobe washed with large quantity of water.

The ion-exchange method is used when M²⁺ and M³⁺ are not stable inalkaline medium or no suitable soluble salt of the anion A^(n−) can befound. An LDHs precursor is first synthesized and the ion-exchangereaction is then carried out in the presence of the required interlayeranions under appropriate conditions in order to prepare the target LDHs.In this method, the washing process cannot be omitted due to theformation of salt by-products in the production of the precursor. WO2007/051377A1 discloses a method for preparing LDHs with interlayerorganic anions containing double bonds, in which an LDHs precursorcontaining interlayer NO₃ ⁻ anions is first synthesized by thecoprecipitation method, and after washing and filtration, LDHscontaining organic anions containing double bonds is obtained by theion-exchange process.

In the calcination-rehydration method, complex metal oxides (LDO) areobtained by calcination of an LDHs precursor, and the LDO is added intoa solution containing the desired anions to restore or partly restorethe ordered layered structure of LDHs. Generally, it is possible torestore the ordered layered structure when the calcination temperatureis below 500° C. When the calcination temperature exceeds 600° C., aspinel phase is formed from which the layer structure of the LDHs cannotbe restored. An LDHs precursor must also be synthesized for use in thecalcination-rehydration method and therefore the washing process cannotbe omitted due to the formation of salt by-products. In a relatedreference (Wei Jiang, LanPing Nong, WenLing Lai, ZeYu Chen,Intercalation and assembly of organic acid radical-pillared layereddouble hydroxides by calcination-rehydration, Chemical Research andApplication, 2004, 16(6): 828-830), LDHs with anions of myristic acid orstearic acid as the interlayer anion were prepared by synthesizingMgAl-LDHs and ZnAl-LDHs precursors with the coprecipitation method,calcinating the precursors to obtain LDO, and then putting LDO inmyristic acid or stearic acid solution. In the process of preparing theprecursors, a large amount of by-product is formed and large quantity ofwater is required for washing.

SUMMARY OF THE INVENTION

An object of this invention is to provide a clean method for preparinglayered double hydroxides. In this method, M²⁺(OH)₂, M³⁺(OH)₃, and CO₂or H_(n)A^(n−) are used as starting materials to produce LDHs byatom-economical reactions.

Most metal hydroxides have low solubility product constants and aredifficult to dissolve in water. Their solubility increases at hightemperatures and pressures. The resulting solution of metal cations canco-precipitate with CO₃ ²⁻ or other anions, giving layered doublehydroxides with the required interlayer anion. The reaction can berepresented by the following overall equation:

(1−x)M²⁺(OH)₂ +xM³⁺(OH)₃ +x/nH_(n)A+(m−x)H₂O→[M²⁺ _(1-x)M³⁺_(x)(OH)₂]A^(n−) _(x/n) .mH₂O

Only M²⁺(OH)₂, M³⁺(OH)₃, and CO₂ or H_(n)A^(n−) are used in this method.Use of NaOH or other materials has been avoided. All the atoms of thereactants are incorporated into the target product. Therefore the atomefficiency of this reaction is 100% which is a typical atom-economicalreaction. In addition, the process of washing and filtration can beomitted because there is no by-product, such that the environment can beprotected and water resource is also saved.

The preparation method of LDHs provided in the present inventionincludes the following steps:

A. Hydroxides of M²⁺ and M³⁺ (with a molar ratio of M²⁺/M³⁺=2˜4) areadded into water (the weight ratio of H₂O to the hydroxides being0.25˜999) to obtain a mixture, and the mixture is then added into areactor, wherein M²⁺ represents one or two divalent cations selectedfrom Mg²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺, Cd²⁺, and Be²⁺, ofwhich Mg²⁺, Zn²⁺, Ca²⁺, and Ni²⁺ are preferable; M³⁺ represents one ortwo trivalent cations selected from Al³⁺, Co³⁺, Fe³⁺, Mn³⁺, Cr³⁺, V³⁺,In³⁺, and Ga³⁺, of which Al³⁺ and Fe³⁺ are preferable.

B. CO₂ is fed into the reactor in such an amount that the molar ratio ofCO₂/M³⁺=1-70, or H_(n)A^(n) (with the molar ratio of M³⁺/A^(n−)=n) isadded into the reactor, and then the contents in the reactor are reactedfor 0.1-10 days to obtain a layered double hydroxide with CO₃ ²⁻ orA^(n−) as the intercalated anion.

In the above step B, CO₂ can be added into the reactor in a form of gasor dry ice, and the molar ratio of CO₂/M³⁺ is preferably 2˜20.

In the above step B, H_(n)A^(n) is an acid containing any of thefollowing anions: (1) inorganic anions: F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, ClO₃ ⁻,ClO₄ ⁻, IO₃ ⁻, H₂PO₄ ⁻, CO₃ ²⁻, S₃ ²⁻, S₂O₃ ²⁻, HPO₄ ²⁻, WO₄ ²⁻, CrO₄²⁻, PO₄ ³⁻ and so on; (2) organic anions: terephthalate, adipate,succinate, dodecyl sulfonate, p-hydroxybenzoate, benzoate and so on; and(3) isopolyacid and heteropolyacid anions: Mo₇O₂₄ ⁶⁻, V₁₀O₂₈ ⁶⁻,PW₁₁CuO₃₉ ⁶⁻, SiW₉V₃O₄₀ ⁷⁻. Among them, Cl⁻, NO₃ ⁻, CO₃ ⁻, SO₃ ²⁻, PO₄³⁻, terephthalate, succinate, benzoate and Mo₇O₂₄ ⁶⁻ are preferred.

The reactor may be a rector fitted with a reflux device, or an airtightreactor fitted with a churn-dasher. In case of the rector fitted with areflux device, the reaction was carried out for 1˜10 days. In case ofthe airtight reactor, the reaction was carried out at a temperature of100˜300° C. and a pressure of 0.1˜10 MPa for 0.1˜3 days.

The obtained layered double hydroxide may be filtered directly after thereaction and dried at 70° C.

According to the powder XRD pattern, infrared spectra, and elementalanalysis, the chemical composition of the product can be determined as[M²⁺ _(1-x)M³⁺ _(x)(OH)₂]A^(n−) _(x/n).mH₂O,

whereinM²⁺ is one or two selected from Mg²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Co²⁺,Fe²⁺, Mn²⁺, Cd²⁺, and Be²⁺;M³⁺ is one or two selected from Al³⁺, Co³⁺, Fe³⁺, Mn³⁺, Cr³⁺, V³⁺, In³⁺,and Ga³⁺;H_(n)A^(n) is an acid containing any of the following anions: (1)inorganic anions: F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, ClO₃ ⁻, ClO₄ ⁻, IO₃ ⁻, H₂PO₄⁻, CO₃ ²⁻, SO₃ ²⁻, S₂O₃ ²⁻, HPO₄ ²⁻, WO₄ ²⁻, CrO₄ ²⁻, PO₄ ³⁻ and so on;(2) organic anions: terephthalate, adipate, succinate, dodecylsulfonate, p-hydroxybenzoate, benzoate and so on; and (3) isopolyacidand heteropolyacid anions: Mo₇O₂₄ ⁶⁻, V₁₀O₂₈ ⁶⁻, PW₁₁CuO₃₉ ⁶⁻, andSiW₉V₃O₄₀ ⁷⁻. Among them, Cl⁻, NO₃ ⁻, CO₃ ²⁻, SO₃ ²⁻, PO₄ ³⁻,terephthalate, succinate, benzoate and Mo₇O₂₄ ⁶⁻ are preferred.x represents the molar ratio of M³⁺/(M²⁺+M³⁺) and is in the range0.2˜0.33;m represents the amount of crystal water and is in the range 0.2˜0.33;n represents the charge of the intercalated anions and is an integer of1˜7.

The present invention is advantageous in that all the atoms of reactantsare converted into the target product and there is no by-product; thepure product can be obtained by drying the target product withoutwashing so that a large amount of water can be saved with considerablebenefit to the environment.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 shows the XRD patterns of MgAl—CO₃-LDHs obtained in example 1.

FIG. 2 shows the FT-IR spectra of MgAl—CO₃-LDHs obtained in example 1.

FIG. 3 shows the TEM photo of Ni₈Fe₂(OH)₂₀(C₈H₄O₄).4H₂O obtained inexample 2.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION Examples Example 1

MgAl—CO₃-LDHs were prepared as follows.

Step A: Total 10 g Mg(OH)₂ and Al(OH)₃ with a molar ratio ofMg²⁺/Al³⁺=2:1 were put into 90 g of deionized water, and the resultingmixture was transferred to a three-necked flask fitted with a refluxdevice.

Step B: The mixture was heated to 100° C. under stirring for four days,while CO₂ was fed thereto at a flow rate of 10 ml/min. The resultingslurry product was filtered and dried in air at 70° C. for 8 h to giveMg₄Al₂(OH)₁₂CO₃.4H₂O.

The powder XRD patterns of the prepared Mg₄Al₂(OH)₁₂CO₃.4H₂O wererecorded using a Shimadzu XRD-6000 diffractometer, and were shown inFIG. 1. The typical peaks which correspond to the (003), (006), (009)reflections were found at 2θ=11.7°, 23.4°, 34.5°. It could be concludedfrom the patterns that the samples had a layered crystal structure.

The infrared spectra of the samples, obtained using a Bruker Vector 22model Fourier transform infrared spectrometer (FT-IR), were shown inFIG. 2. The infrared strong absorption peak at 1361 cm⁻¹ could beattributed to the symmetric vibration absorption of CO₃ ²⁻, which wasintercalated between the MgAl-LDH layers. No absorption peak arisingfrom impurities were observed in FIG. 2.

Example 2

Ni₈Fe₂(OH)₂₀(C₈H₄O₄).4H₂O was prepared as follows.

Step A: Total 5 g Ni(OH)₂ and Fe(OH)₃ with a molar ratio ofNi²⁺/Fe³⁺=4:1 were put into 90 g of deionized water, and the resultingmixture was transferred to a three-necked flask fitted with a refluxdevice.

Step B: 0.58 g terephthalic acid was added to the flask. The contents inthe flask were heated to 100° C. and reacted for eight days understirring. After filtration and drying in air at 70° C. for 8 h,Ni₈Fe₂(OH)₂₀(C₈H₄O₄).4H₂O was obtained.

The TEM image of the product produced in example 2, obtained using aHitachi S-3500N scanning electron microscope, was shown in FIG. 3. Theimage reveals a layered hexagonal morphology of LDHs.

Example 3

ZnMg₃Al₂(OH)₁₂CO₃.4H₂O was prepared as follows.

Step A: Total 20 g Zn(OH)₂, Mg(OH)₂ and Al(OH)₃ with a molar ratio ofZn²⁺/Mg³⁺/Al³⁺=1:3:2 were put into 80 g of deionized water, and themixture was transferred to an airtight reactor fitted with achurn-dasher.

Step B: After adding 40 g of dry ice to the reactor, the system washeated at 150° C. for one day under stirring. The resulting slurry wasfiltered and dried in air at 70° C. for 8 h to giveZnMg₃Al₂(OH)₁₂CO₃.4H₂O.

Elemental analysis of the product obtained in example 3 was performed onan ICPS-7500 inductively coupled plasma spectrometer. The molar ratio ofZn:Mg:Al was measured as 1:3:2 and no Na⁺ was found in the product.

Example 4

Ca₄Al₂(OH)₁₂CO₃.4H₂O was prepared as follows.

Step A: Total 20 g Ca(OH)₂ and Al(OH)₃ with a molar ratio ofCa²⁺/Al³⁺=3:1 were put into 80 g of deionized water, and the mixture wastransferred to an airtight reactor fitted with a churn-dasher.

Step B: The system was heated to 250° C. under stirring, while carbondioxide gas was flowed into the reactor to keep the pressure at 5 Mpafor 0.5 day. The resulting slurry was filtered and dried in air at 70°C. for 8 h to give Ca₄Al₂(OH)₁₂CO₃.4H₂O.

Example 5

Mg₆Fe₃(OH)₁₈(PO₄).4H₂O was prepared as follows.

Step A: Total 4 g Mg(OH)₂ and Fe(OH)₃ with a molar ratio ofMg²⁺/Fe³⁺=2:1 were put into 600 g deionized water, and the mixture wastransferred to an airtight reactor fitted with a churn-dasher.

Step B: 0.58 g H₃PO₄ was put into the reactor. The contents in thereactor were heated to 100° C. for 1.5 days under stirring. Theresulting slurry was filtered and dried in air at 70° C. for 8 h to giveMg₆Fe₃(OH)₁₈(PO₄).4H₂O.

1. A clean method for preparing layered double hydroxides comprising thefollowing steps: A. hydroxides of M²⁺ and M³⁺ with a molar ratio ofM²⁺/M³⁺=2˜4 are added into water to obtain a mixture in which the weightratio of H₂O to the hydroxides is 0.25˜999, and the mixture is thenadded into a reactor, wherein M²⁺ represents one or two divalent cationsselected from Mg²⁺, Zn²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺, Cd²⁺, andBe²⁺, and M³⁺ represents one or two trivalent cations selected fromAl³⁺, Co³⁺, Fe³⁺, Mn³⁺, Cr³⁺, V³⁺, In³⁺, and Ga³⁺; and B. CO₂ is fed insuch an amount that the molar ratio of CO₂/M³⁺=1-70 into the reactor, orH_(n)A^(n) is fed in such an amount that the molar ratio of M³⁺/A^(n−)=ninto the reactor, and then the contents in the reactor are reacted toobtain a layered double hydroxide with CO₃ ²⁻ or A^(n−) as anintercalated anion, wherein H_(n)A^(n) is an acid containing any of thefollowing anions: (1) inorganic anions: F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, ClO₃ ⁻,ClO₄ ⁻, IO₃ ⁻, H₂PO₄ ⁻, CO₃ ²⁻, SO₃ ²⁻, S₂O₃ ²⁻, HPO₄ ²⁻, WO₄ ²⁻, CrO₄²⁻, and PO₄ ³⁻; (2) organic anions: terephthalate, adipate, succinate,dodecyl sulfonate, p-hydroxybenzoate, and benzoate; and (3) isopolyacidand heteropolyacid anions: Mo₇O₂₄ ⁶⁻, V₁₀O₂₈ ⁶⁻, PW₁₁CuO₃₉ ⁶⁻, andSiW₉V₃O₄₀ ⁷⁻, and n represents the charge number of the anions and is aninteger of 1˜7.
 2. The method according to claim 1, wherein: in step A,M²⁺ represents one or two divalent cations selected from Mg²⁺, Zn²⁺,Ca²⁺ and Ni²⁺, and M³⁺ represents one or two trivalent cations selectedfrom Al³⁺ and Fe³⁺; and in step B, A^(n−) is one selected from Cl⁻, NO₃⁻, CO₃ ²⁻, SO₃ ²⁻, PO₄ ³⁻, terephthalate, succinate, benzoate and Mo₇O₂₄⁶⁻.
 3. The method according to claim 1, wherein CO₂ is fed into thereactor in form of gas or dry ice, and the molar ratio of CO₂/M³⁺ is2-20.
 4. The method according to claim 1, wherein the reactor is areactor fitted with a reflux device, and the contents in the reactor arereacted under stirring for 1˜10 days.
 5. The method according to claim1, wherein the reactor is an airtight reactor fitted with achurn-dasher, and the contents in the reactor are reacted under stirringfor 0.1˜3 days at a temperature of 100˜300° C. and a pressure of 0.1˜10MPa.